The difference is that 0.5% on the whole payment, might in some cases be actually much more than taxes on the profit. Or even worse, if you sell for whatever reason without profit, the tansaction fee is your loss.

Standard VAT in Poland is 23% so taxes usually dwarf fees.

0.5% gross on a 1% margin business (which is most retail) is egregious.

I would like to know where that 1% retail business is located. My family has a bunch of hardware/electricity retail businesses and the margins are way more than that. Like 30%-500%.

Groceries and fuel come to mind.